1. Field of the Invention
The present invention relates to a stabilized power supply circuit, and more particularly to a stabilized power supply circuit that is suitable for a charge pump power supply circuit.
2. Description of the Related Art
A charge pump power supply circuit is one of known power supply circuits. FIG. 1 is a circuit diagram showing a typical example of a charge pump called a doubler, As shown in FIG. 1, the charge pump includes four switching elements 1, 2, 3, and 4, and a single capacitor 5. The charge pump doubles an input voltage Vin, which is an output voltage of a power source 6, to thereby obtain an output voltage Vout.
Specifically, when the switching elements 1 and 2 are turned on at a certain period, the input voltage Vin is applied to the capacitor 5, whereby the capacitor 5 is charged. During this period, the switching elements 3 and 4 are maintained off. As a result, the voltage between the opposite ends of the capacitor 5 increases to the input voltage Vin. In a subsequent period, the switching elements 3 and 4 are turned on, and the switching elements 1 and 2 are turned off. As a result of this operation, the input voltage Vin is superposed on the voltage (Vin) between the opposite ends of the capacitor 5 via the switching element 4, whereby the voltage between the opposite ends of the capacitor 5 becomes two times the input voltage Vin, and is output via the switching element 3 to an output terminal 7, as the output voltage Vout.
In general, each of the switching elements 1 to 4 is formed of a MOSFET, and is switched by means of clock pulses output from a clock pulse generation circuit (not shown). Further, a smoothing capacitor 8 is provided in order to remove pulsation components from the output voltage Vout to thereby stabilize the same.
In such a charge pump power supply circuit, although the output voltage Vout is stabilized by means of the smoothing capacitor 8, its stability is insufficient; e.g., the output voltage Vout drops when large load current flows, In other words, essentially, the above-described charge pump power supply circuit does not function as a switching regulator or stabilized power supply circuit.
In order to solve the above-described problem, there has been proposed a stabilized power supply circuit which utilizes a charge pump power supply circuit whose output voltage Vout is stabilized. FIG. 2 shows the proposed stabilized power supply circuit. As shown in FIG. 2, in the stabilized power supply circuit, a voltage representing the output voltage Vout (in this example, a voltage obtained through division of the output voltage Vout by resistances R1 and R2 of resistors 9 and 10) is compared with a predetermined reference voltage Vref to thereby obtain an error signal corresponding to the difference therebetween. The thus-obtained error signal is used to control the on resistance of the switching element 4, which is formed of a MOS transistor. That is, through control of the on resistance, the voltage that is superposed on the voltage of the capacitor 5 via the switching element 4 is controlled so as to maintain the output voltage Vout at a constant level.
More specifically, when the switching element 4, together with the switching element 3, is turned off by means of a clock pulse of the charge pump (the switching elements 1 and 2 are turned on), a switching element 11 is turned on, and a switching element 12 is turned off.
Meanwhile, when the switching element 4 is turned on, the switching element 11 is turned off, and the switching element 12 is turned on by means of the clock pulse. As a result, the error signal, which is the output voltage of an error amplifier 13, is applied to the gate of the switching element 4 via the switching element 12. However, the on resistance of the switching element 4 varies in accordance with the applied voltage. That is, the on resistance of the switching element 4 is determined in accordance with the deviation, and a voltage corresponding to the on resistance (the smaller the on resistance, the higher the voltage) is superposed on the voltage of the capacitor 5, whereby a stepped up voltage is output as the output voltage Vout.
In this manner, the output voltage Vout is maintained at a constant level, whereby the charge pump circuit functions as a switching regulator or stabilized power supply circuit.
Although the stabilized power supply circuit shown in FIG. 2 can stabilize the output voltage Vout of the charge pump power supply circuit, the circuit has the following problems.
First, the switching element 4 is driven by means of a voltage corresponding to the error signal and through changeover of the switching elements 11 and 12, and thus, the output current of the switching element 4 varies abruptly, whereby the capacitor 5 is charged abruptly. Current stemming from the charging flows toward the output terminal 7 via the switching element 3 and is superposed on the output current as a ripple. As a result, the quality of the output voltage of the stabilized power supply circuit deteriorates. Second, since the error amplifier 13 must drive the switching element 4, which serves as a load thereof, at constant voltage, the load of the error amplifier 13 increases, and thus, the error amplifier 13 must be of large capacity.
Moreover, the stabilized power supply circuit shown in FIG. 2 encounters a problem of wasteful consumption of electrical power in the case where, irrespective of its load state (the magnitude of load), the switching elements 1, 2, 3, and 4 are always turned on and off at predetermined intervals by means of clock pulses so as to achieve charge pump operation. That is, when the stabilized power supply circuit is caused to achieve charge pump operation, it naturally consumes electrical power. However, in a charge pump of the above-described type, the switching elements 1, 2, 3, and 4 of relatively large capacity must be turned on and off, so that the consumed electrical power increases accordingly. For example, in the case where charge pump operation is continuously performed even in a standby mode of a camera to which electrical power is required to be supplied instantaneously in the form of a pulse, a large amount of electrical power is wastefully consumed by the charge pump operation during a greater portion of the standby period in which the instantaneous electrical power is not required.